Pandemic influenza

At irregular intervals, an influenza A virus emerges which is different from the current human seasonal influenza viruses and can not only infect humans but can also cause disease in some of them and crucially is capable of efficient human to human transmission. The virus has to be novel enough to prevail over the seasonal A viruses, and because of its novelty there can be little specific immunity among humans, except for older people who may have met a similar virus in the past. This new virus can then spread rapidly from human to human all over the world. Because of the lack of human immunity the virus causes a variable amount of severe disease and deaths: this is an influenza pandemic. Read more facts about pandemic influenza